This invention relates to electroscopic printing powders useful for developing latent electrostatic images produced by photoelectrostatic copying techniques into visible material images. More particularly, the invention relates to electroscopic powders which have been formulated with low molecular weight, high density polyolefinic resins which improve fixability of the powders onto copy sheets through the use of pressure.
Photoelectrostatic copying processes in which a photoconductive medium is imaged to produce a differential electrostatic charge which is then developed with an electroscopic powder are well known. A wide variety of photoconductive media may be employed such as inorganic photoconductive insulating crystalline metal ion containing materials, organic photoconductors and elemental photoconductors.
A wide variety of techniques are known for developing the differentially charged photoconductive medium such as magnetic brush, powder cloud, liquid development and cascade developing techniques.
The formulation of electroscopic powders to be compatible in a particular photoelectrostatic copying environment has been widely explored in this art and is well developed. For the most part, the powders are applied by the various techniques mentioned hereinabove and ultimately the powder image requires fixing so that it will adhere to the copy material. The copy material may be photoconductive itself, such as in the case of zinc oxide coated paper. In other processing systems, the powder image is first produced on a photoconductive drum and then transferred to a sheet of plain paper where the powder image must be fixed. In either case, the electroscopic powder requires that it be permanently fixed to the material which is to become the permanent copy.
In the copying systems disclosed heretofore, the techniques of fixing the image onto the copy depended on the use of heat in order to fuse the thermoplastic resin powder onto the copy material. The use of heat energy is generally acceptable, but it is not without serious deficiencies. For example, the equipment requires that it be warmed up to an operating temperature level where the heating system will properly fuse the powder. The presence of heat presented the hazard of igniting the papers in the circumstance that there is a paper jam in the paper delivery system, and at the very least was known to char the papers.
In terms of the design of the equipment the use of heat required provision for large power inputs to the equipment which made it costly to manufacture and maintain.
Another undesirable aspect of using heat to fuse the powder images is the introduction of heat into the working environment causing some discomfort.
One important consideration is the time which is required to impart sufficient heat to the thermoplastic powder so that it will properly soften and coalesce. In most copying systems, the rate of output of reproductions is only as fast as the slowest processing step, which heretofore was the heat fusing operation.
Electroscopic powders have been suggested which are formulated especially for pressure fusing. It has been found, however, that they tend to produce reproductions whose images are either of poor resolution or have poor image densities unless extremely high pressure are used.
It is taught in U.S. Pat. No. 3,775,326 to Virgil W. Westdale assigned to the same assignee as this invention that by compounding the prior art toners which were primarily heat fusible with a polyolefinic resin in the range of 2% to 15% based on the weight of resin, these formulations then became uniquely and surprisingly responsive to pressures in the range of 300 pounds per lineal contact inch so that they could be permanently bonded to the image receiving surface.